POTENTIOMETERS

Abstract: A potentiometer less liable to produce transient voltage spikes caused by its contacts being lifted from the track of electro-resistive material when encountering detritus or dust uses slider contacts having divergent contact areas. Such areas, and the contiguous sliders, function as ploughs which sweep the detritus out of the path of the slider. This may be achieved by using contacts of rectangular-sectioned precious metal which may be twisted about their longitudinal axis so that an edge of the contact first comes into abrasive contact with the track, so that a roughly-triangular or rhomboidal flat (20) is worn on the contact. For very abrasion resistant contact materials, the flat may be pre-formed before the contact is located on the track.

Description: POTENTIOMETERS This invention relates to potentiometers, and particularly to rotary and linear potentiometers constructed as individual components for use in electrical and electronic circuits. Potentiometers in general are electrical devices which have a known voltage established across the ends of a length (track) of electro-resistive material. The voltage at any point along the length of the track may be picked off, detected or otherwise used by means of a slider in low-resistance contact with the track. In rotary potentiometers, the track is arcuate, and the slider is moved by means of a rotary knob or other control member so that the voltage of the slider is a function of the angular displacement of the slider from a reference position. In linear potentiometers, the track is straight. In some low-voltage potentiometers, the track takes the form of a layer of electro-resistive material of uniform composition and transverse cross-sectional area along its length. In sliding, and therefore frictional, contact with the layer is at least one strip of precious metal having its end up turned to produce a bend, the outside surface of which is in contact with the layer across the width of the strip. The friction between the layer and the slider results in material being abraded from either or both in use. The abrasion products become deposited on the track, with the result that, with time, the slider bumps over them, coming out of electrical contact with the track. This results in the production of transient voltages which lead to electrical noise in the associated electrical circuitry having the voltage of the slider as its input. Many attempts have been made to improve the performance of potentiometers by modifying the way in which the slider contacts the track, for example as described in DE-A-4226380, US-A-4928038 and GB patent specifications 824234, 828755, 1122145 and 1498608. Most of these suggestions materially increase the cost and need for precise assembly of the potentiometer components. The present invention aims at reducing the production of electrical noise in potentiometers, by designing the slider so that it sweeps abrasion products out of its path over the track. Accordingly in a first aspect the present invention provides a potentiometer comprising at least one track of electro-resistive material adapted to be contacted by at least one relatively-movable slider contact, in which the or each slider is made from a length of wire or strip of non-circular cross-section, the slider having an upturned end forming a bend in a plane, with part of the convex surface of the bend having had material removed from it to form a contact surface which presents at least one apex directed substantially in at least one direction of movement of the slider relative to the track, which contact surface is substantially parallel to the track and wherein the median plane of the bend is not normal to the plane of the track. The contact surface may be pre-formed on the or each slider prior to assembly of the potentiometer or, as described below, formed by wear after assembly. The slider may be bent at its end in a plane or may be bent and twisted slightly, e.g. by bending a flat strip slider about a bending tool edge not set at right angles to the length of the strip. More generally, the invention provides in order to sweep abrasion products from the track as the slider is moved in at least one direction, a potentiometer having a slider in contact with a track along which it may move, wherein the contact surface is a flat having a peripheral contour with at least one apex pointing in a direction of movement of the slider, the apex intersecting a ridge on the body of the slider and not lying in the plane of the flat. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a side elevation of one form of rotary potentiometer of the present invention; Figure 2 is an end elevation of the potentiometer shown in Figure 1; Figure 3 is a scrap view, drawn to an enlarged scale, showing one form of slider contact in accordance with the present invention; Figure 4 is a scrap view, drawn to a larger scale, of the part encircled in Figure 2, showing the use of three parallel sliders acting in concert on a single track; Figure 5 is a cross-sectional view of the three sliders, track and substrate drawn to a larger scale than Figure 4; Figure 6 is a plan view of the contact area of each slider of Figure 5, after it has been worn down to some extent; Figure 7 is a plane view of a slider carrier according to a preferred form of the invention, and Figure 8 is an end view of the slider carrier of Figure 7. In that form of the invention shown in Figure 1, the basic components are a fixed base 2 of dielectric material having on it a track 4 of electro-resistive material applied to it by a suitable deposition process, as is already known. Rotatable about an axis 6 of the potentiometer is a control body 8, also of dielectric material, having a metal support 10 secured to it, by a screw 12 or other suitable means. Projecting from end of conductor 10 are three sliders, each made from a strip of precious metal (e.g. Paliney alloy, ex the J M Ney Co., Inc.) having an upturned end to form a part-cylindrical surface of which one end is to function as the contact face between the slider and track 4. In practice, each end of track 4 would be connected through electrical conductors to static terminals fast with substrate 2, and across which a known potential can be applied. The conductor 10 is also connected to a third terminal of which the voltage varies in accordance with the angle by which the body 8 has moved about axis 6 relative to substrate 2. In that form of the invention shown in Figure 2, the body may be moved in either of the two angular directions, with the track 4 being split into two halves so that each set of sliders 14 picks up the same adjustable voltage. The present invention aims at providing a slider having a contact area with the track presenting at least one apex in one angular direction. In one embodiment of the invention, as shown in Figures 3 to 6, the desired shape of the contact area is achieved by forming the slider 14 from a relatively-flat strip 16 of gold or other precious metal not liable to have its surface oxidised to produce high-resistance surface films. As shown most clearly in Figure 3, at its end remote from the conductor 10 the end of strip 16 is formed into a rounded V-section, ensuring that both major surfaces of the V-section meet the surface of the track 4 at substantially-equal angles. In accordance with one embodiment of the present invention, the strip 16 is deformed about its longitudinal axis so that the outside surface of the bend produced by the V- section 18 comes into contact with surface 4 only at one end, and not across the width of the strip 16. Although the edge of bend 18 may be polished before being assembled in position, to produce a 'flat' 20 (shown more clearly in Figure 6) where the side face 22 of strip 16 meets the bend 18, it has been found that the flat 20 is produced naturally after a few traverses of the track. In this way the strip 14 beds down naturally to form the flat 20 which is absolutely parallel with the initial surface of track 4, so that the slider comes into smooth contact with the track over a relatively-large area 20. With particular reference to Figure 6, the movement of track 4 relative to area 12 takes place in the tangential direction indicated by the double-headed arrow 24. It will thus be seen that the roughly-triangular area 20 in effect has two apices each pointing in the direction of the path which it sweeps out over the surface of track 4. Each apex functions in the manner of a plough to push to one or other side of the apex any abrasion products or dust which it encounters as it is sliding over the surface of the track. This ploughing effect thus pushes such detritus to both sides of the path swept out by the slider, so that there is very little tendency, if any, for the detritus to become jammed under the slider. This thus reduces or eliminates any tendency for such detritus to lift surface 20 from the track. As shown in Figures 4 and 5, it is often convenient to use three sliders 14 in parallel engaging the same track 4 so that even if one slider does encounter some detritus which lifts it out of contact with the track, the other sliders stay in contact and conduct the respective voltage on the surface of the track to the respective terminal connected to conductor 10. Although the roughly-triangular contact surface 20 can be achieved by imparting a torsional distortion or twist to strip sliders, it is within the purview of the present invention to provide similar or more-effective contact areas by appropriate selection of the cross-sectional shape and size of the material making up the slider. It is envisaged that a substantially-square section wire could be used in place of strip 16, with the wire being positioned so that one edge of the wire would initially tend to come into contact with the track. This edge would become polished or otherwise abraded to form a rhomboidal contact surface. With such a surface being substantially symmetrical about the path 24 of relative movement of the track, the maximum distance any integral piece of detritus would have to be moved would be only half the width of the path swept by the slider, whereas that embodiment shown in Figures 3 to 6 in particular has its apex positioned well to one side of the centre line of the path, which might make sweeping aside the detritus less efficient. Turning now to Figure 7 and 8 these show a preferred form of the slider carrier for use in the potentiometer of Figures 1 and 2. Conductor 10 has two apertures 30 through which screws or other suitable fixing means pass when the potentiometer is assembled. At each end of the conductor spot welded onto conductor 10 are three sliders 40. Instead of being twisted (which is rather complex to manufacture) in contrast to the slider shown in Figures 3 to 5, each slider 14 is bent around two fold lines 32 and 33 respectively. Folds are easier to control in manufacturing than twisting and as can be seen, the end of each slider 14 is first of all bent upwards and then bent downwards. In other words, as Figure 7 is viewed, the folds at fold line 32 are valley folds and at fold line 33 ridge folds. This provides the three sliders on each side of conductor 10 lying at different levels from the plane of conductor 10 as clearly seen in Figure 8, when the component is manufactured, but the angle of mounting of conductor 10 on the control body 8 and the relative position of the fixed base 2 with the electro-resistive track 4 on it are such that the three folds along fold line 33 on each side come to lie, when the potentiometer is assembled, resiliently against the track 4. In order that the contact pressure does not vary materially between the three sliders 14, the edge of conductor 10 is angled at 36, as shown on Figure 7. While not necessary, it is customary, and lengthens the service life of the potentiometer, to provide a little lubricant material on the track. Various types of contact lubricant are known in the potentiometer art, and most may be used, though any which leads to a building of an insulating layer on surface 20 should be avoided. Generally silicone type lubricants are preferred, particularly if the conductive track is of a silicone polymer/carbon black composition. For example, silicone oil type DC200, ex Dow Corning, can be used. Silicone lubricants have the additional advantage, generally of operating satisfactorily over a very wide ambient temperature range, e.g. -40 to +130°C. It will be appreciated that although the invention has been specifically described as being applied to rotary potentiometers, it is equally applicable to linear potentiometers having a track of electro-resistive material engaged by at least one slider. Accordingly it will be seen that the present invention provides an easily-implemented method and apparatus for making a potentiometer less liable to produce electrical noise.

Claims: CLAIMS 1. A potentiometer comprising at least one track of electro-resistive material adapted to be contacted by at least one relatively-movable slider contact, in which the or each slider is made from a length of wire or strip of non-circular cross-section, the slider having an upturned end forming a bend, with part of the convex surface of the bend having had material removed from it to form a contact surface which presents at least one apex directed substantially in at least one direction of movement of the slider relative to the track, which contact surface is substantially parallel to the track and wherein the median plane of the bend is not normal to the plane of the track. 2. A potentiometer as claimed in Claim 1, in which the slider is made from a length of strip of rectangular cross-section, the slider being twisted about its longitudinal axis from its fixed end whereby one boundary of the contact area is defined by an edge of the strip. 3. A potentiometer as claimed in Claim 1, in which the slider is made from a length of wire of a cross-sectional shape having at least one apex, in which the wire is oriented so that the edge formed by the apex is at the bottom of the curve in the slider, whereby the contact area of the slider is substantially symmetrical about the direction of relative movement of the slider on the track. 4. A potentiometer having a slider in contact with a track along which it may move, wherein the contact surface is a flat having a peripheral contour with at least one apex pointing in a direction of movement of the slider, the apex intersecting a ridge on the body of the slider and not lying in the plane of the flat.

Inventors: CAIRNS MICHAEL PAUL, GOODINGE JOHN WILLIAM DEVEREUX, STAINES COLLIN LESLIE, CAIRNS, MICHAEL, PAUL, GOODINGE, JOHN, WILLIAM, DEVEREUX, STAINES, COLLIN, LESLIE
Applicants: CAIRNS MICHAEL PAUL, COLVERN LTD, GOODINGE JOHN WILLIAM DEVEREUX, STAINES COLLIN LESLIE, CAIRNS, MICHAEL, PAUL, COLVERN LIMITED, GOODINGE, JOHN, WILLIAM, DEVEREUX, STAINES, COLLIN, LESLIE
Application year: 1994
Main classification: H01C1
IPC codes: H01C1, H01C10